docs(migrations): add async/await example (#11231)

docs/migrations.md

@@ -285,6 +285,57 @@ module.exports = {
 
 ```
 
+The next is an example of a migration that has uses async/await where you create an unique index on a new column:
+
+```js
+module.exports = {
+  async up(queryInterface, Sequelize) {
+    const transaction = await queryInterface.sequelize.transaction();
+    try {
+      await queryInterface.addColumn(
+        'Person',
+        'petName',
+        {
+          type: Sequelize.STRING,
+        },
+        { transaction }
+      );
+      await queryInterface.addIndex(
+        'Person',
+        'petName',
+        {
+          fields: 'petName',
+          unique: true,
+        },
+        { transaction }
+      );
+      await transaction.commit();
+    } catch (err) {
+      await transaction.rollback();
+      throw err;
+    }
+  },
+
+  async down(queryInterface, Sequelize) {
+    const transaction = await queryInterface.sequelize.transaction();
+    try {
+      await queryInterface.removeColumn(
+        'Person',
+        'petName',
+        {
+          type: Sequelize.STRING,
+        },
+        { transaction }
+      );
+      await transaction.commit();
+    } catch (err) {
+      await transaction.rollback();
+      throw err;
+    }
+  },
+};
+```
+
 ### The `.sequelizerc` File
 
 This is a special configuration file. It lets you specify various options that you would usually pass as arguments to CLI. Some scenarios where you can use it.